Chemical (Aptasia X) or Lemon Juice in a syringe, squirt when he is open and into his mouth ( center)
Eradicate these pests and never let them back in. They will multiply and both fish and corals.
Pull them, sure, but they will retract fast to a very small pimple, so, hard to get this way.
Every time you see one, whack it!
Inspect all things going into the tank to try and prevent entry.
Some people use a peppermint shrimp (me included) as a long term control, but they have been known to bother anenomes as well.